1. Field of the Invention
The present invention relates to the lubrication of diesel engines run on biodiesel fuels and to the stabilization of the lubricating oil against degradation induced by the biodiesel fuels.
2. Description of the Related Art
In an effort to reduce the dependency on petroleum-based hydrocarbon fuels, various renewable sources of fuels have been identified and investigated.
Diesel fuels, traditionally petroleum hydrocarbon fractions or synthetic hydrocarbon fractions such as hydrocarbons derived from Fischer-Tropsch processes and boiling in the distillate boiling range of are viewed as non-renewable resource fuels, being produced either from crude oil or, by various synthetic reactions, from natural gas.
Lubricating oils containing one or more phenolic antioxidants and/or one or more aromatic amine antioxidants and/or zinc dialkyl diphenylamine are known in the literature.
EP 1,878,784 teaches a long life fuel saving engine oil composition comprising a mineral and/or synthetic base oil, an amine antioxidant and a phenolic antioxidant, and molybdenum dithiocarbamate. The oil is described as exhibiting excellent oxidative stability at high temperatures. In addition to the components recited above, the lubricating oil composition may also contain other additives such as detergent, zinc dialkyl dithiophosphate, ashless dispersants, VI improvers, pour point depressants, metal deactivators, rust preventors and anti-foaming agents. In the Examples, formulations containing phenolic antioxidants, amine antioxidants, MoDTC and further containing ZDDP are presented. The formulation exhibited excellent oxidation stability in the Sequence IIIG test.
U.S. 2006/0223724 teaches a lubricating oil of reduced phosphorus levels which retains excellent viscosity control; i.e., excellent oxidation stability. The oil comprises a major amount of one or more of a Group II, Group III, Group IV and synthetic ester base stock, 4,4′ methylenebis (2,6 ditertbutyl phenol), an alkylated diphenyl amine, a detergent and zinc dialkyldithiophosphate. Optionally an oil soluble organomolybdenum compound can be present, as can additional, different hindered phenolic antioxidants. The lubricant contains about 600 ppm or less phosphorus derived from the ZDDP. In the Examples, a preblend was prepared consisting of a 150N Group II base oil, an ashless dispersant, an overbased detergent, a neutral detergent and a secondary zinc dialkyldithiophosphate. To the preblend various other components were added individually, including various hindered phenols and aromatic amines. A number of examples contain all three of ZDDP, a hindered phenol and an aromatic amine.
U.S. Pat. No. 6,300,292 teaches an hydraulic oil of excellent oxidative stability comprising a vegetable oil base oil and at least one antioxidant selected from the group consisting of a phenol antioxidant, an amine antioxidant and a zinc dithiophosphate antioxidant in an amount of 0.01 to 5 wt % based on the total composition. The vegetable oil can be rapeseed oil, sunflower oil, soybean oil, corn oil, canola oil, mixed oil. The vegetable oil has a total degree of unsaturation of 0.3 or less and an oleic acid content of not less than 70% by mass in triglyceride-constituting fatty acids.
The antioxidants can be used individually or as a combination of two or three of such components. In a mixture of three components the ratio by weight may be 1:1:1. There are no examples of any formulation containing all three suggested antioxidant types, let alone as a premix.
EP 1,006,173 teaches a lubricant exhibiting extended oxidation stability comprising a hydraulic oil anti-wear component and a base oil and an additive comprising an amine antioxidant and at least one additional antioxidant selected from ashless dithiocarbamate, sulfurized olefin and phenolic antioxidant. The base oil can include animal oils and vegetable oils. The hydraulic oil anti-wear additive is hydraulic grade zinc dialkyldithiophosphate. Formulations containing ZDDP, a phenolic antioxidant and one or more aminic antioxidants are reported. It does not appear that a premix was employed.
EP 0,896,050 teaches a lubricating oil comprising a particular base oil and an oxidation inhibitor. The base oil is a mineral oil-derived lubricant having a saturated hydrocarbon content of 80% or more by mass and a viscosity-density constant of the saturated hydrocarbon component of 0.79 or less. The antioxidant is selected from the group consisting of hindered phenolic, aromatic aminic and sulfur containing antioxidants. Sulfur containing antioxidants include zinc dihydrocarbyl dithiophosphate. The antioxidant can be a single material or a mixture of two or more components in any ratio. There were no examples containing antioxidants from all three categories in a single formulation.
EP 0,725,130 teaches a lubricating oil highly resistant to oxidation by nitrogen oxides. The lubricant consists of a hydrocarbon oil base oil, molybdenum dithiocarbamate, zinc dithiophosphate and a phenol antioxidant. Additional additives can be present, including amine-based antioxidants. There were no examples of a lubricant containing an amine antioxidant in addition to the zinc dithiophosphate and phenol antioxidant.
“Additive Interactions and Depletion Processes in Fuel Effluent Engine Oils”, Johnson, Millen, et al., SAE Technical Paper Seven 971694, May 5-8, 1997. This paper investigates the interaction between molybdenum dithiocarbamate and zinc dialkyldithiophosphate and phenolic antioxidants. The presence of the antioxidants inhibits oxidation caused by peroxy radicals by trapping the radicals and reducing the rate of oxidation. The use of additional presence of aminic antioxidants is not mentioned or discussed.
JP 09 272882 teaches a hydraulic oil comprising a mineral oil having an aromatic content of 1.5 mass % or less and dialkyl zinc dithiophosphate, 2,6 di-tert-butyl-4 methyl phenol and P-P′ dioctyl diphenyl amine.
JP 59 041388 teaches the stabilization of coal liquefaction oil by the addition of a radical stabilizer containing hindered phenol, aromatic sec- or tert-amine and metallic dithiophosphate. The stabilizer is added to the oil prior to heating and prevents coking.
Biodiesel fuels have been identified as an alternative to conventional distillate fuels.
Biodiesel fuels are based on the trans-esterification of triglyceride of fatty acids secured from vegetable oils and animal fats. Trans-esterification of the vegetable oils and animal fats into fatty acid alkyl esters is necessary to reduce the viscosity of the vegetable oils and/or animal fats to more closely resemble that of traditional diesel fuel.
Vegetable-based biodiesel is currently the more common of the biodiesel fuels. Vegetable oils used as base stock sources include soy, rapeseed, palm, cottonseed, peanut, sunflower, coconut, canola, etc. while animal fats include lard, tallow, fish oil, poultry fat, etc.
Trans-esterification employs alcohols in the presence of a catalyst. The alcohol can be any C1 to C5 alkyl alcohol, but, for the sake of economy, methanol is the most widely used alcohol. The biodiesel is commonly identified by reference to its plant source. Thus there exist soy alkyl ester, rapeseed alkyl ester, cottonseed alkyl ester, etc. fuels, typically soy methyl ester (SME) and rapeseed methyl ester (REM).
It is readily apparent, therefore, that such alkyl ester fuels are mixtures of molecules of various molecular weight with ester functionality and often with one or two double bonds in the alkyl group associated with the fatty acid starting materials.
Such ester functionalities and olefinic double bonds are chemically active groups making biodiesel fuel chemically and kinetically unstable, causing both the biodiesel fuel itself and the hydrocarbon-based lubricating oil to oxidize, prematurely resulting in sludge and deposit formation in the engine.
Despite this susceptibility to early oxidation, biodiesel is an attractive fuel source because in comparison to conventional diesel fuel, biodiesel fuels are derived from renewable sources and exhibit improved performance in CO2, CO, hydrocarbon and particulate matter emissions while having at least equivalent or in some instances superior cetane numbers.
Thus it would be an advance if a way could be found to reduce or eliminate the harmful early oxidation of the diesel lubricant associated with the use of biodiesel fuel.